1. Field of the Invention
The present invention relates to deriving information about an AC power source in an image formation apparatus.
2. Description of the Related Art
A zero crossing signal indicates a half-wave switching timing of an AC voltage waveform, and is used for a phase control and the like. An electric appliance to which a power is supplied from an AC power source, such as an image formation apparatus, uses the zero crossing signal in order to control turning on/off of a fixing heater. Conventionally, the following methods are widely used as a method of generating the zero crossing signal that is used for such control:    (1) A method of generating a zero crossing signal by a zero crossing signal detecting circuit that is a combination of a rectification circuit and a photocoupler;    (2) A method of generating a zero crossing signal by a zero crossing signal detecting circuit that is a combination of a transformer and a photocoupler.
In the image formation apparatus, the zero crossing signal is generated at the zero crossing signal detecting circuit, and sent to a controller such as a CPU. Upon receiving the zero crossing signal, the controller generates an interruption. Then, in the interruption processing, the controller controls turning on/off of the fixing heater for every half cycle of a power source frequency.
Specifically, at a time when the zero crossing signal is input, the controller generates a signal for turning off a triac that is a device turning on/off the fixing heater, that is, a signal for turning off the fixing heater. The controller further starts a timer, and generates a timer interruption in a few milliseconds after the time of the zero crossing signal input. When the timer interruption signal is generated, the controller generates, within the timer interruption, a signal for turning on the triac, that is, a signal for turning on the fixing heater.
In the image formation apparatus, the controller generates an interruption by using the zero crossing signal in this manner, and turns on and off the triac, thereby to control the turning on/off of the fixing heater.
In order to carry out the phase control of the fixing heater in the image formation apparatus, the zero crossing signal must be detected beforehand. Therefore, the image formation apparatus decides whether the zero crossing signal is detected in the initialization processing immediately after the power source of the image formation apparatus is turned on.
Specifically, in the zero crossing detection processing, when the controller generates a zero crossing interruption, the image formation apparatus decides that the zero crossing signal is generated. On the other hand, when the controller does not generate the zero crossing interruption, the image formation apparatus decides that the zero crossing signal is not generated. In this case, the image formation apparatus displays on an operation unit that the zero crossing signal is not generated. Furthermore, the image formation apparatus starts a one-second interruption timer, and counts the number of zero crossing interruptions generated until the next timer starts to count up to decide whether the power source frequency is 50 Hz or 60 Hz. For example, when the timer counts 45 to 54 times of zero crossing interruptions, the image formation apparatus decides that the power source frequency is 50 Hz, and when the timer counts 55 to 64 times of zero crossing interruptions, the image formation apparatus decides that the power source frequency is 60 Hz. On the other hand, when the timer counts 0 to 44 times or more than 64 times of zero crossing interruptions, the image formation apparatus displays on the operation unit that the zero crossing signal is erroneously detected.
As explained above, an electric appliance such as the image formation apparatus carries out control by using a zero crossing signal. Therefore, when the zero crossing signal is erroneously detected, the electric appliance becomes malfunctioning, thus it is necessary to prevent the occurrence of the abnormal detection. However, when the zero crossing signal detecting circuit generates the zero crossing signal by constantly monitoring the voltage waveform of the AC power source as with the methods (1) and (2), the zero crossing signal is detected erroneously when the power source voltage waveform is disturbed for some reason.
For example, when the AC power source voltage varies as shown at the bottom of the graph in FIG. 14, a zero crossing is detected at a time when the power source voltage is disturbed due to a noise or the like. As a result, a zero crossing signal is generated as shown at the top of the graph in FIG. 14.
When the image formation apparatus is used in the environment where a power source of a private power generator is used or in the environment where a number of large-power apparatuses are used, there has been a problem that a noise affects the power source, and a zero crossing is detected erroneously. In this case, since the detection of the power source frequency cannot be performed normally with an error message displayed, it is not possible to use the image formation apparatus.
To cope with the problem, when the image formation apparatus is used in the noisy environment, a technique has been proposed such that a power source frequency used in the image formation apparatus is obtained from a user, without carrying out a processing of detecting the power source frequency. Then, the obtained power source frequency from the user is stored in a ROM as a fixed frequency, and the ROM is fitted to the image formation apparatus.
Another method proposed to reduce such an erroneous detection of the zero crossing signal is that a zero crossing timing is decided based on conditions whether a time during which a power source voltage is within a predetermined range (i.e., a range around zero) is longer than a set value and whether the polarity of the power source voltage before entering the range is inverted after exiting the range (for example, refer to Japanese Patent Application Laid-open Publication No. 8-308215).
There is also a proposed technique of generating a zero crossing signal by detecting a starting point and an ending point of a half-wave of a voltage waveform that is supplied from the AC power source (for example, refer to Japanese Patent Application Laid-open Publication No. 2002-268450).
When using the technique of storing the power source frequency information in a ROM, however, the information stored in the ROM is not always correct, and the power source frequency may vary depending on the environment. In this case, it is not possible to accurately detect the zero crossing point.
Likewise, when using the technique of deciding whether the obtained voltage satisfies a predetermined condition (refer to Japanese Patent Application Laid-open Publication No. 8-308215) and the technique of detecting a starting point and an ending point of a half-wave (refer to Japanese Patent Application Laid-open Publication No. 2002-268450), although it is possible to exclude the influence of the noise, the voltage waveform must always be monitored, which increases a processing load.